Rheumatoid Arthritis (RA) is a chronic and progressive autoimmune disorder that affects about one percent of the US population (1). Existing therapies treat the symptoms of the disease but not the underlying cause, and are associated with numerous side effects (2). The activity of Protein Arginine Deiminase 4 (PAD4), one of four known active PAD isozymes, is increased in RA; where it is thought to generate a subset of antigens that the immune system recognizes as foreign (3). Genetic, serological, and biochemical evidence suggests that dysregulated PAD4, and potentially PAD2, activities play a role in both the onset and progression of RA (1). Cl-amidine, a compound that specifically inactivates PAD4, reduces disease severity and incidence in the collagen-induced model of arthritis (CIA) (unpublished observations). However, because Cl-amidine inhibits all of the PAD isozymes with equipotency, it is unclear whether the observed reduction in disease severity is due to the inhibition of single or multiple PADs. This is particularly relevant because both PAD 2 and 4 are overexpressed in the joints of patients with RA (4). Thus, the identification of PAD selective inhibitors would facilitate the characterization of their individual contributions to the onset and progression of RA and represent a promising novel therapeutic approach for RA.

Assay Overview:The purpose of this assay is to assess the potency of test compounds for inhibition of PAD4 using a substrate-based assay. In this assay, the target enzyme is pre-incubated with test compound followed by addition of substrate Na-benzoyl-L-arginine ethyl ester HCl (BAEE). The percent activity remaining is determined by measuring the amount of citrulline produced using a standard colorimetric absorbance assay. Test compounds that act as PAD4 inhibitors will prevent the production of citrulline. IC50 values for inhibition of recombinant PAD4 were determined from dose-response curves from 2 trials at each inhibitor concentration in a 7-point dilution series from 0 to 15 uM.Protocol Summary:Recombinant PAD4 (0.2 uM) in assay buffer (50 mM NaCl, 10 mM CaCl2, 2 mM DTT, 100 mM Tris-HCl, pH 7.6) was treated with test compound (0-15 uM final concentration; stock in DMSO) for 15 minutes at 37 C. Substrate BAEE (10 mM) was added, and the reaction was incubated for 15 minutes at 37 C in 60 uL total reaction volume. The reaction was quenched by flash freezing in liquid nitrogen, and a color-developing reagent for detection of the enzymatic byproduct citrulline (COLDER; 200 uL), which consists of solution A (80 mM diacetyl monoxime and 2 mM thiosemicarbazide) and solution B (3 M H3PO4, 6 M H2SO4, and 2 mM NH4Fe(SO4)2) in a 1:3 ratio, was added. This mixture was incubated for 30 minutes at 95 C and the absorbance was measured at 540 nm. The amount of product produced was determined by comparison to a standard curve with known concentrations of citrulline.%_Inhibition = 1 - ( ( ABStest - Blank ) / Correction_Factor ) / ( ( ABSDMSO - Blank ) / Correction_Factor)Where:ABStest is defined as absorbance at 540 nm for target treated with test compound.ABSDMSO is defined as absorbance at 540 nm for target treated with DMSO.Blank is defined as the absorbance of a blank sample at 540 nm.Correction_Factor is defined as the correction factor generated from a standard curve of known concentrations of citrulline.The percent activity remaining was fit to the following equation using GraFit (version 5.0.11):Fractional_Activity = 1 / ( 1 + ( Conc / IC50 ) )Where:Conc is the concentration of inhibitor.IC50 is the concentration of inhibitor that yields half-maximal activity.PubChem Activity Outcome and Score:Compounds with IC50 values less than or equal to 1 uM were considered active. Compounds with IC50 values less than 1 uM were considered inactive.The PubChem Activity Score range for inactive compounds is 0-0. There are no active compounds.List of Reagents:Recombinant PAD4 (supplied by Assay Provider)Tris HCl (Sigma, part T3038)CaCl2 (Sigma, part C3881)DTT (RPI, part D110000)2,3-butanedione monooxime (Sigma, part B0753)Thiosemicarbazide (Sigma, part T33405)NH4Fe(SO4)2 (Sigma, part F1668)H2SO4 (Sigma, part 258105)H3PO4 (Fisher, part A260)NaCl (Sigma, part S6546)BAEE (Sigma, part B4500)96-well plates (BD Falcon, part 353228)